1. Technical Field
The present invention relates generally to a video project apparatus implemented with a spatial light modulator. More particularly, the invention relates to a spatial light modulator implemented with plate lines for transmitting signals to modulate said pixel array.
2. Description of the Related Art
Even though there are significant advances made in recent years on the technologies of implementing electromechanical micro-mirror devices as spatial light modulator, there are still limitations and difficulties when employed to provide high quality images display. Specifically, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with sufficient number of gray scales.
Electromechanical micro-mirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micro-mirror devices. In general, the number of required devices ranges from 60,000 to several million for each SLM. Referring to FIG. 1A, an image display system 1 including a screen 2 is disclosed in a relevant U.S. Pat. No. 5,214,420. A light source 10 is used to generate light beams to project illumination for the display images on the display screen 2.
The light 9 projected from the light source is further concentrated and directed toward lens 12 by way of mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate the light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. FIG. 1B shows a SLM 15 that has a surface 16 that includes an array of switchable reflective elements 17, 27, 37, and 47, each of these reflective elements is attached to a hinge 30. When the element 17 is in an ON position, a portion of the light from path 7 is reflected and redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge on the display screen 2 to form an illuminated pixel 3. When the element 17 is in an OFF position, the light is reflected away from the display screen 2 and, hence, pixel 3 is dark.
The on-and-off states of the micromirror control scheme, as that implemented in the U.S. Pat. No. 5,214,420 and in most conventional display systems, impose a limitation on the quality of the display. Specifically, applying the conventional configuration of a control circuit limits the gray scale gradations produced in a conventional system (PWM between ON and OFF states), limited by the LSB (least significant bit, or the least pulse width). Due to the ON-OFF states implemented in the conventional systems, there is no way of providing a shorter pulse width than the duration represented by the LSB. The least intensity of light, which determines the gray scale, is the light reflected during the least pulse width. The limited levels of the gray scale lead to a degradation of the display image
Specifically, FIG. 1C exemplifies, as related disclosures, a circuit diagram for controlling a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a based on a Static Random Access switch Memory (SRAM) design. All access transistors M9 on a Row line receive a DATA signal from a different Bit-line 31a. The particular memory cell 32 is accessed for writing a bit to the cell by turning on the appropriate row select transistor M9, using the ROW signal functioning as a Word-line. Latch 32a consists of two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states that include a state 1 when is Node A high and Node B low, and a state 2 when Node A is low and Node B is high
The control circuit positions the micro-mirrors to be at either an ON or an OFF angular orientation, as that shown in FIG. 1A. The brightness, i.e., the number of gray scales of display for a digitally control image system, is determined by the length of time the micro-mirror stays at an ON position. The length of time a micromirror is in an ON position is controlled by a multiple bit word. FIG. 1D shows the “binary time intervals” when controlling micromirrors with a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8, which in turn define the relative brightness for each of the four bits where “1” is the least significant bit and “8” is the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales for showing different levels of brightness is a represented by the “least significant bit” that maintains the micromirror at an ON position.
For example, assuming n bits of gray scales, one time frame is divided into 2n−1 equal time periods. For a 16.7-millisecond frame period and n-bit intensity values, the time period is 16.7/(2n−1) milliseconds.
Having established these times for each pixel of each frame, pixel intensities are quantified such that black is a 0 time period, the intensity level represented by the LSB is 1 time period, and the maximum brightness is 2n−1 time periods. Each pixel's quantified intensity determines its ON-time during a time frame. Thus, during a time frame, each pixel with a quantified value of more than 0 is ON for the number of time periods that correspond to its intensity. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analog levels of light.
For controlling deflectable mirror devices, the PWM applies data to be formatted into “bit-planes”, with each bit-plane corresponding to a bit weight of the intensity of light. Thus, if the brightness of each pixel is represented by an n-bit value, each frame of data has the n-bit-planes. Then, each bit-plane has a 0 or 1 value for each mirror element. According to the PWM control scheme described in the preceding paragraphs, each bit-plane is independently loaded and the mirror elements are controlled according to bit-plane values corresponding to the value of each bit during one frame. Specifically, the bit-plane according to the LSB of each pixel is displayed for 1 time period.
Meanwhile, higher levels of resolution and higher grades of gray scales required for better quality display images are in demand for projection apparatuses, especially in recent years due to the increased availability of video images, such as that provided by high definition television (HDTV) broadcasting.
However, in the gray scale control by the pulse width modulation (PWM), as shown in FIG. 1D, the expressible gray scale is limited by the length of the time period determined by the LSB. An attempt to add a new control structure to a memory cell of the above described SRAM structure in order to overcome the aforementioned limitation creates another problem, that is, the structure of a complex memory cell, with a larger number of transistors than, for example, the memory cell of a DRAM structure, increases the size of the mechanism.
Specifically, in order to obtain a higher definition display image, a large number of mirror elements are required. Each of these mirror elements, comprising an SRAM-structured memory cell, must be reduced in size to fit in the space of a certain mounting size (e.g., a predefined package size or chip size). However, the addition of a new control structure to an SRAM-structured memory cell in order to attain a higher level gray scale display image increases the size of the memory cell, thereby inhibiting a higher level display image.
In light of the above described limitations, it is necessary to solve the technical challenge of realizing a higher gray scale level, which exceeds the conventional method to control the gray scale a display image with pulse width modulation (PWM), while projecting images with a higher definition with memory cells having the simplest possible structure comprising a small number of circuit elements.